Process for treating plywood



Patented May 7, 1946 No Drawing. Application August 15, 104

Serial No. 407,009

9 Claims. (cl. iii-s09) This invention relates to an improved process for treating plywood, and has special reference to a process for relieving the stresses in hot pressed plywood so as to prevent subsequent checking and warping thereof.

Hot pressed plywood is manufacturedby applying heat and pressure to plywood veneers arto 12% being a desirable range. In the process referred to, treatment at the elevated temperature and high humidity requires from fourteen to thirty-six hours to produce the desired moisturecontent, depending upon the thickness of the individual panels. This is due to the fact that in the high temperature atmosphere the hot panels ranged in gluing relationship. Such'a plywood panel usually consists of three, live or seven sheets of wood'veneer superposed, one upon the other, with the grain of each succeeding sheet running at right angles to the grains of adjacent sheets. The bonding material used for hotv pressed plywood is a thermo-setting glue such as a dry glue,

or glue having a resin base, whose bondingproperties are eifected at elevated temperatures.

when a resin base glue is used the panels are introduced between the heated platens of the hot plate press and subjected to temperatures ranging from 240 to 350? F, during the pressing operation, the period of time varying with the absorb moisture very slowly. when the desired moisture content has been'reached, the panels must be cooled gradually to a temperature not above 150 F. before they can be removed from the kiln. a

The process Just described has met with considerable success in relieving stresses in hot pressed plywoodpanels, but leaves much to be desired as a satisfactory routine process in industry. In the-firstv place, the process is time consuming, requiring, as stated, from fourteen to thirty-six hours for the heat treatment alone,

so before the cooling can commence. 'The process thickness of the panel beingpressed. High pressure contact of the heated platens with the plywoodfaceshe'ets during the hot pressing operation drives any moisture in the panel into the interior thereof, away from thesurface, and produces a surface hardeningeflect analagous to.

the case hardening of metals.

When the panels, which have been pressed at temperatures within the range described, are removed from the hot plate press, their surfaces are exceedingly low in moisture content and are at the same timeextremely hot. If panels having such hot and desiccated surfaces are exposed to the atmosphere. considerable checking will result from the shrinkage attending the rapid cooling of the surfaces 0n the other hand, if the panels are allowed to cool in piles or stacks protected from the atmosphere, checking will be held to a minimum, but upon subsequent exposure to the atmosphere many stresses will be set up in the panels-because of unequal absorption. of moisture from the air, and serious warping problems result. This result obtains when the panels are closely stacked to excludethe atmosphere and only the edges of the panels are exposed for moisture absorption.

It has been proposed to heat treat hot pressed plywood panels in a hot, humid atmosphere on.

mediately after the hot pressing operation and also is costly, as it requires a considerable amount of heat energy to maintain the kilns with their charges and high water vapor content at sustained elevated temperatures. Also, as each charge must becooled in the kiln, the kiln must be reheated for each new charge. In addition, the long time required for carrying out-the process on each charge necessitates a large number of kilns to handle the output of the hot presses in a large plywood plant. Furthermore, the proccan itself is a delicate one to manage, particularly in the cooling of! stage. Coordinating the reduction in humidity along with the reduction in temperature in the cooling stage presents a troublesome problem requiring close observation, and prolongs the cooling period. Y

It is, therefore, an object of this invention-to provide an improved process for relieving stresses in hot press plywood panels to prevent checking and warping.

Another object is to provide a process for treating hot press plywood which will materially reduce the normalizing time. 7

A further object is to provide a normalizing process requiring fewer kilns and less expensive equipment than is at present required.

A further object is toprovide' a normalizing process less costly in operation than a process which requires the additionof heat energy.

before the panels have cooled. In one such proccos a temperature range of'from 150 to 210 F. is employed, with a relative humidity-of 98%. The panels remain in the kiln until the moisture content. thereof is raised to 6% or higher, '6% Additional objects and advantages It stillfurther object is to provide a-simple and inexpensive process for handling plywood panels as removed from a hot plate press so that the plywood product will have many improved chameterlstics and qualities.

of the inand the wood veneer core sheets and face sheets are placed together to make the type of panel desired. The application of considerable heat and great pressure is then required to flatten out the plies of veneer and secure an even and continuous bonding of the several plies which are to make a finished panel. This heat and pressure is supplied in the hot press, the temperature being between 240 and 350 F. for panels using a resin base glue. Under these conditions the several veneer sheets are united into a smooth plywood panel.

An undesirable effect of this exposure to high temperature is produced on the surfaces of the panel, where the wood is abnormally dried, resulting in a condition analogous to case hardening in metals. The surface cells and fibers are found to be so materially altered by contact with the hot platens that they have lost much of their original elasticity or resilience, and. will no longer accommodate themselves, without tearing, to stresses produced during cooling after removal from the press. since an abnormal drying condition does not have time to penetrate deeply into the panel, the case hardening, as with metals, is largely a surface phenomenon, and does not appear to an objectionable degree in core veneers inside of the face sheets. ,In fact, a secondary action of the heated platens has been observed to be the driving of the moisture inwardly, often in the form of steam, into the core veneers, having the eifect of somewhat plasticizing the central portions of the panel and reacross the entire surfaces of the panels. The high moisture content of the air may be effected by a system of nozzles arranged to spray cold water into-the air in very fine particles, producing a thick fog or mist. The water particles are held mechanically. in suspension in the atmosphere by a system of blower fans which cirwhite the moisture laden air rapidly across the surfaces of the panels.

As the particles of moisture in the atmosphere first contact the hot surface of the panel, they are further volatilized into a low temperature vapor product. This action, of course, takes place only at the immediate surface of the panels, and

lieving all stresses in said central portions at i the sametime that the component parts are being held flattened by the press.

This invention is predicated. then, upon the concept that it is largely a surface condition which must be corrected by a-successful normalizing process. For this reason, it is the teaching of this invention that a further heat treatment is unnecessary. The process of the invention involves exposing the surfaces of the hot plywood panels, after they have been removed from the hot press, to a relatively cold circulating atmosphere containing moisture in such an amount that it is capable of supplying moisture to the panel'surfaces at a rate at least as great as the possible rate of moisture absorption by the panels. It is found that the tendency of the surface fibers to shrink from cooling is completely neutralized by the tendency of the panel surfaces to expand from moisture absorption. By the time the panels have cooled to normal atmospheric temperature, the moisture content will have increased suinciently to have effected a complete and permanent normalizing, so that checks, cracks and warping will'not occur.

In practice, the process of this invention is preferably carried out by taking the panels after removal from a hot plate press and placing them in chambers or kilns where. they are subjected to a supersaturated atmosphere. The phrase "supersaturated atmosphere" is intended to convey the meaning that moisture is present in an amount greater than that which can be carried in the air in solution at the temperature of the kiln. Thepanelstobetreatedarestackedin suitable racks with spacer sticks therebetween,

and the moisture laden atmosphere is circulated serves further to relieve the stresses in the face veneers produced by so-called case hardening during subjection to heat and pressure in the hot plate press. After the panels have cooled to a temperature below 212 F., they rapidly absorb moisture from the wet atmosphere, the rate of moisture absorption being relatively great at first and decreasing gradually until the temperature of the panel approaches. the temperature of the atmosphere within the kiln. The water content of the layer of atmosphere next adjacent the panels is preferably sufficient that moisture is supplied to the panels at a rate at least as great' as the possible rate of moisture absorption by the panels. In this way the panels are cooled without danger of surface checking. The panels may be-allowed to remain'in the kiln and in the circulating moisture ladenatmosphere for an additional hour, or thereabouts, after they have reached the minimum temperature of the circulating atmosphere itself. For panels not exceeding one-fourth inch in thickness, the total time in the kiln does not necessarily have to exceed two hours.

The quantity of hot panels introduced into the kiln determines the temperatures at which the kiln operateasince heat is not added in any other form. While in normal operation the air temperature within the kiln is between and F., the exact temperature of the kiln is, of course, affected by the outside temperature, so that the temperature range of operation may be considerably greater than that stated. In cold winter weather, the kiln'temperature may go down to 50 F. or colder, and in hot summer weather the kiln temperature may go as high as F. or higher. It will be apparent, therefore, that the lower the kiln temperature the greater must be the volume of water carried in suspension in the atmosphere in order to prevent checking of the face veneers during cooling. In the kiln the panels cool quite rapidly from contact with the colder atmosphere, creating a tendency for the face veneers, or at least the surface portions thereof, to shrink. This tendency. however, is neutralized by the tendency toward expansion of the face veneers because of the absorption of moisture by the wood itself from the supersaturated atmosphere.

The advantages of this process are manifold. A much less expensive installation is required for carrying out this process than for any normalizing process previously proposed. For example, the normalizing time of the plywood is greatly reduced. thereby greatly increasing the output of any particular installation. The operating cost of equipment required in the practice of this invention is greatly reduced because the high moisture content of the air is produced by the use of cold water instead of steam. and it is v75 not necessary to apply external heat as ordinarily for maintaining temperatures in the Hot panels delivered by the hot plate press. when cooled by the process of'this invention, are free from surface checking, and contain approximately two to five percent increase in moisture content over their moisture content when leaving the hot plate presses. They are pliable and flat and remain so even when subsequently subiected to various unfavorable atmosphericconditions. a

Broadly, this invention involves the application of moisture to the surfaces of hot press plywood panels during the cooling of the same, without heat treatment, and it is preferred to disperse the water into the atmosphere in the form of a mist or heavy fog, and blow. this water-laden atmosphere over the surface of the panels. The fog-forming method for applying moisture to the surfaces of the panels has been found to be very eflective and is preferred because of its simplicity and adaptability to treating large quantities of plywood. Conveying the moisture to the spaced panels by blowing water laden atmosphere thereover has been found to give greater uniformity ortreatment and greater ease of control than other methods. The water content of the atmosphere in the kiln, for example, may be conveniently, accurately and automatically controlled by merely installing a moisture-responsive device to regulate a valve in the water pipe line supplying the spray nozzles.

It is found that it is not necessary to p rate the sprays continuously throughout the normalizing process. The sprays are operated continuously, however, imtil the panels have cooled to substantially below 150' I". It will be understood that they may be operated continuously if it is foundn in order to obtain an adequate no. The moisture requirements for normalizing are, of course, variable, depending upon the kind of wood, the thickness of the veneers,

the thickness of the panels, and the time and temperature variables of the hot press operation. Moisture may be applied to the panels by bodily dipping them into cold water for a predetermined interval of time, or by sponging or spraying the panels, and satisfactory normalizing has been effected in these ways, but these methods do not so readily lend themselves to industrial production involving a large output as doesthe preferred method of dispersing the water in the form of a fog and circulating the fog across the surfaces of the panels. I The term normalizing is used in this speciiication, and in the appended claims, to describe,

in general, any process following the hot press operation which is designed to relieve stresses in the plywood panels which cause checking, cracking and warping. A normalizing process is considered to make a panel'normal in that it removes the latent tendency of the panel to depart from its smooth and flat condition.

Baving-now the process of my invention and in what manner the same may be prac ticed, what I'claim as new and desire to protect by Letters Patent is:

1. The method of normalizing hot pressed plywood panels which comprises taking said panels while still in a heated condition from the hot pressing operation and placing them in spaced apart relationship in a clnmber, the atmosphere ofsaidchamberbeingatatemperature notsubstantially higher than-the prevailing atmospheric ticles in the atmosphere of said chamber, and

and normalise said panels.

2. A controlled surface stress relieving process for normalizing hot pressed plywoodv panels which comprises placing said plywood panels while still in a heated condition in a kiln having an atmospherictemperature not substantially higher than the prevailing atmospheric temperature and in which water above the saturation point is carried in minute particles mechanically in suspension, and blowing said water. particles over the surfaces of said panels until said panels have cooled to a temperature substantially below 150 1''.

3. The normalizing process for relievingthe stresses in hot pressed plywood panels which comprises exposing the'surfaces of the heated panels to atmosphere at substantially normal atmosipheric temperature, passing water into said at- .4. The method of normalizing hot pressed plywood panels which comprises exposing the surfaces of said panels while still in a heated condition from the hot pressing operation to atmosphere. spraying water into said atmosphere in an amount that the moisture content of said atmosphere is in excess of. the saturation point thereof, and maintaining said moisture content in the atmosphere until said panels have cooled to substantially the temperature of said atmosphere.

5. The process of normalizing hot pressed plywood panels compoad of a plurality of bonded to-' gether wood plies with adjoining plies having the grains thereof running at an angle to each other which comprises introducing the panels while still in a heated condition from the hot pressing operation into a humid atmosphere at a temperature not substantially higher than F. until the panels absorb an appreciable amount of moisture, introducing water into said atmosphere to compensate for water absorbed by said panels, and circulating said atmosphere over the surfaces of said panels until they are cooled to substantia'lly the temperature'of said atmosphere.

6. The method of normalizing hot pressed plywood panels, which comprises placing said panels .while' still 'in a heated conditionfrom, the hot particles in the atmosphere of said kiln, and circulating said water laden atmosphere over the surfaces of said panels to cause said surfaces to tend to swell from increased moisture content simultaneously with their tendency to shrink from cooling.

'7. The process of normalizing hot pressed plywood panels which comprises, placing said panels while still in a heated condition from the hot pressing operation in 'an lmheated chamber, spraying cold water into said chamber for producing a water laden atmosphere in said chamber having a temperature not substantially higher than 120 l"., and circulating the water laden atmosphere in said chamber over the surfaces of said panels until the panels absorb an appreciable amount of moisture and have cooled to substantially the temperature of the atmosphere in said chamber.

8. The method of normalizing hot pressed plywood panels which comprises, placing said panels while still in a heated condition from the hot pressing operation in a chamber of substantially normal atmospheric temperature, spraying water into said chamber for producing a moisture laden iievins suriace stresses in hot pressed plywood panels which comprises introducing said panels while still in a heated condition from the hot pressing operation into an unheated atmosphere. introducing water into said atmosphere to produce an atmosphere in which water is carried in suspension for adding moisture to said panels, and

circulatins' said atmosphere across said panels until said panels have cooled to a temperature 10 substantially below 150 1".

MICHEL PASQUIER. 

